Waste-treatment systems may operate over a wide area, drawing sewage from numerous collection points to one or more central facilities for treatment. Offensive and even toxic gaseous emissions may be generated throughout the system wherever complete containment is impossible. For example, typical municipal waste-treatment systems utilize gravity flow to transport sewage to the central facility. In systems that serve even moderately sized areas, it is impractical to conduct waste along a single stretch of conduit from the most remote collection point to the treatment center; the declivity between end points of the conduit that would be necessary to maintain adequate flow would be too great (requiring, for example, locating the discharge point at the treatment center many feet underground). Instead, the sewage path is divided into segments short enough to require only a few feet of incline. Between segments, "lift stations" increase the height of the flowing material so that the declivity between stations can be roughly the same.
A lift station may be visualized as a large containment vessel within which the volume of liquid is periodically changed. Gas dissolved or generated in the liquid as well as vapor from the liquid rise from its surface within the vessel, and as the liquid level rises, the effect is like that of a piston: the gas is compressed and vessel pressure increases. To avoid damage to such vessels, which ordinarily cannot tolerate large pressures, some of the gas must be discharged. Odor control is therefore important. Typical systems employ filters, scrubbers, oxidation systems, and/or biosystems that reduce the concentration of offensive or environmentally deleterious material in the gas. See, e.g., U.S. Pat. No. 5,354,545. Such systems may be complex and require high throughput capacity to efficiently process large volumes of gas discharged as the liquid level rises in the vessel.
The problem of odor containment is not restricted to systems in which liquid levels vary. In waste-digestion systems, for example, internal pressure may increase as a result of the biological processes utilized to treat the waste, which create gaseous byproducts. The volumes of gas generated may vary substantially over the treatment cycle, necessitating frequent venting and, as a result, the use of filtration systems capable of processing the peak levels in order to maintain constant internal pressure.